There is a growing economic need for precursors, having applications in OMCVD processes to fabricate alkaline earth metal (i.e. barium, strontium and the like) containing materials, that are cleanly volatile sources for such metals. To be useful, such a precursor must be able to deliver a steady gas phase transport of metal-containing vapor during the CVD process to deposit a thin film of the metal or metal-containing compound onto a substrate. It is especially important that the vapor pressure of the metal precursor is stable over time to permit precise control over the elemental composition in the final product.
Examples of alkaline earth containing materials produced in a CVD process are high temperature superconducting (HTSC) ceramic thin films being developed for use in microelectronic devices. In a typical high temperature superconductor (HTSC) thin film CVD process, a vapor containing "volatile" organometallic compounds of barium, yttrium and copper, e.g. bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato) barium(II) [Ba(HFA).sub.2 ], tris(2,2,6,6-tetramethyl-3,5-heptanedionato)yttrium(III) [Y(DPM).sub.3 ], and bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)copper(II) [Cu(HFA).sub.2 ], is contacted with oxygen over a SrTiO.sub.3 substrate at 10 Torr pressure and a temperature of 600.degree. C. to deposit an amorphous thin film. After further annealing in air for three hours at 850.degree. C., a high temperature superconducting ceramic thin film is produced with the composition YBa.sub.2 Cu.sub.3 Oy; see K. Shinohara et al Japanese Journal Appl. Phy., Vol. 27, No. 9, pp. L1683-L1685 (1988) and S. Oda et al, Japanese Journal Appl. Phy., Part 2, Vol. 28, No. 3, pp. L427-L429 (1989). The use of the fluorinated barium precursor results in the initial formation of BaF.sub.2 which has to be subsequently annealed with water vapor and/or air to produce the desired oxide phase. To produce directly high quality single crystal or epitaxial mixed oxide films, it is important to avoid the initial formation of BaF.sub.2 to enable the growth of the mixed Y-Ba-Cu oxides. It is therefore desirable to have a precursor that is non-fluorinated for CVD fabrication of HTSC mixed oxide thin films or other barium oxide-based films containing no fluoride. As previously stated, it is also desirable to have a precursor that is cleanly volatile, i.e. volatilizes without decomposition of the source, for accurate control over extended time periods of the stoichiometry of the CVD deposited barium oxide containing thin film.
The two most commonly utilized strategies for preparing volatile barium precursors are the use of bulky or fluorinated ligands that effectively screen barium ions from each other within the molecular structure of the complexes. This is thought to reduce intermolecular associations between barium centers and hence increase their volatility or at least reduce the intermolecular associations to the point where some vaporization is possible. This general phenomenon has been widely exploited in the preparation of volatile metal complexes in general; see R. E. Sievers, Science, Vol. 201, No. 4352, pp. 217-223, (1978). Due to its large ionic radius and propensity towards large coordination spheres, barium is a particularly difficult element to totally screen in this way and "molecularly encapsulate" to yield monomeric compounds. Bulky ligands such as bis(2,2,6,6-tetramethyl-3,5-heptanedionato) barium (II) (thd) and similar .beta.-diketonates (including fluorinated .beta.-diketonates) are limited in their ability to completely supply the degree of coordinate saturation required to achieve monomeric barium complexes. However, even though this deficiency can lead to complexes that are unstable towards sublimation (see A. P. Purdy et al, Inorg. Chem., Vol. 28, pp. 2799-2803 (1989)) there are reports of using bulky and fluorinated .beta.-diketonate barium complexes and "Ba(thd).sub.2 " in CVD processes; see Y. Hianori et al, Supercond. Sci. Technology Vol. 2, pp. 115-117 (1989) and A. J. Panson et al, Appl. Phys. Lett., Vol. 53, No. 18, pp. 1756-8 (1988). Similarly, there are other barium complexes that contain simple ligands (i.e. methyl or ethyl groups) that are also unable to supply the degree of shielding required to yield monomeric complexes; see M. J. McCormick et al., Organometallics, Vol. 8, No. 8, pp. 2044-2049 (1989). This can lead to polymeric or highly associated and hence sometimes involatile compounds of the type (Ba.sup.+2).sub.x (L.sup.-).sub.y where y=2x. With y=2-10, volatile character can be expected but when y&gt;10 polymeric involatile compounds result. Under conditions of strong heating the structure of these polymeric species may break down and in the process release molecular fragments that are more volatile. However this kind of "volatility" is uncontrolled and yields a vapor of constantly changing chemical composition which is unsuited to CVD processes.
Currently, the non-fluorinated barium precursor most widely used for CVD purposes is bis(2,2,6,6-tetramethyl-3,5-heptanedionato) barium(II), more commonly known and incorrectly formulated (vide infra) as "Ba(thd).sub.2 " or "Ba(tmhd).sub.2 " or "Ba(DPM).sub.2 ". However, notwithstanding the popular use of this material as a volatile precursor, it is known to be thermally unstable at CVD source delivery temperatures and to not be cleanly volatile, i.e. the composition of the vaporized material and its vapor pressure vary as a function of time. This causes difficulties in the control of the composition of the deposited film, and therefore in obtaining the desired physical and chemical properties.
It has been concluded there are problems in the OMCVD deposition of YBa.sub.2 Cu.sub.3 O.sub.y caused by the high vaporization temperature of "Ba(thd).sub.2 " precursor and its limited thermal stability; see T. Nakamori et al, Japanese J. Appl. Phys., Vol 27, pp. L1265-L1267 (1988). The authors point out a solution to the problem would be an organic metal chelate having a lower vaporization temperature and a higher thermal stability. The performance of Ba(dipivaloylmethanate).sub.2, "Ba(DPM).sub.2 " or "Ba(thd).sub.2 ", for OMCVD has been criticized for its low volatility, substantial decomposition during transport, lack of vapor pressure reproducibility and low deposition rates; J. Zhao et al, Amer. Appl. Phys. Lett., Vol 53, No. 18, pp.1750-1752, (1988). It has been observed qualitatively that barium diketonates, such as "Ba(thd).sub.2 ", are hygroscopic, with coordinated water removed under mild conditions prior to their use as OMCVD precursors, and that there is a need to avoid exposure to moisture following anhydrous synthesis; see A. D. Berry et al, J. Mater. Res., Vol. 5, No. 6 (June, 1990).
One technique for improving the volatility of barium compounds that do not cleanly sublime without decomposition is to add small neutral molecules to the metal centers of these compounds to form volatile adducts. This helps to prevent ligands being shared between metal centers and hence promotes monomeric nature and volatility. Examples of this approach include the use of tetrahydrofuran (THF) and 1,2-dimethoxy-ethane (diglyme) to increase the volatility of both magnesium and zinc .beta.-diketonates; S. B. Miller et al., U.S. Pat. No. 4,501,602 and R. C. Fay et al, U.S. Pat. No. 4,558,144.
Recently TNO (a division of Technology for Society. Dept Chemistry, P.O. Box 108, 3700 A. C. Zeist, Utrechtseweg 48, Zeist, The Netherlands) disclosed in EPO Patent Application 90201485.1 the preparation of volatile derivatives of barium .beta.-diketonates by treating parent complexes with various linear polyether glymes such as tetraglyme. A problem with this kind of approach is that under heating conditions the added ligands tend to dissociate from the parent complex thereby precluding vaporization of the adduct.
Excess ligand can be added in the form of a saturated carrier gas to suppress dissociation of the adduct and this has been shown to be effective for "Ba(thd).sub.2 " when adding either excess the ligand (P. H. Dickinsen et al, J. Appl. Phys., Vol. 66, pp. 444 (1989)) or excess simple amine; see A. Barron, MRS Symp. Proc., Fall Meeting, Boston, November 1990.
According to the above P. H. Dickerson et al reference, the partial thermal decomposition of "Ba(thd).sub.2 " during transport may be avoided by using an inert carrier gas pre-saturated with organometallic ligand H(thd), which achieves stable precursor vaporization over a period of "several hours".
The stable delivery of "Ba(thd).sub.2 " has been achieved over a period of 80 minutes using a carrier gas enriched with H(thd) as proposed by F. Schmaderer et al., Applied Surface Science, Vol. 46, pp. 53-60(1990).
Tetrahydrofuran has been added to the inert carrier gas which reportedly improves the volatility of "Ba(thd).sub.2 "; see S. Matsuno et al, Japanese J. Appl. Phys., Vol. 29, pp. L947-L948 (June, 1990) and Chern et al, Appl. Phys. Lett., Vol. 57, pp. 721-723 (Aug., 1990). The problems associated with this technique include the unknown chemical identity and changing nature of the gas phase species. If a substantial pressure of added ligand is needed then the CVD processing cannot be accomplished at low pressures thereby losing the inherent advantages of operating in this regime; K. K. Schuegraf, "Handbook of Thin Film Deposition Processes and Techniques", Noyes Publication, 1988, Chapter 3. In addition, since oxygen gas is introduced into the system to effect the oxide deposition, there exists a danger of forming an explosive gas mixture within the CVD reactor since the added ligands are typically flammable or combustible.
A recent publication reports on the study of bis(2,2,6,6-tetra-methyl-3,5-heptanedionato)-tris(methanol)barium, monomethanol, [Ba(C.sub.11 H.sub.19 O.sub.2).sub.2 (CH.sub.3 OH).sub.3.CH.sub.3 OH], and one of its derivatives used as a barium precursor in OMCVD; A. Gleizes et al, C. R. Acad. Sci. Paris, Vol. 312, II, pp. 983-988 (March, 1991).
The foregoing prior art OMCVD precursor compositions (a) are reportedly stable for short time periods, or (b) require the addition of an extra stage to the precursor delivery to achieve additional stability control, or (c) incorporate an excess ligand (e.g. amines/ammonia) which can lead to safety/flammability problems. Such prior art complexes are in contrast to complexes that are in effect composed of non-polymeric discreet "clusters" of barium atoms (i.e. y=2-10) and that are cleanly volatile, undergo no chemical decomposition upon sublimation and require no addition of adduct forming ligand to help in sublimation. Such a compound should be highly suited to CVD processes, especially at low pressures. It is evident from a reading of the number of prior art references in this area that a long heart felt need exists for such a stable, cleanly volatile barium precursor for OMCVD.